1. Field of the Invention
The present invention relates to a handover type judgement scheme for a CDMA (Code Division Multiple Access) mobile communication system which is capable of carrying out any of multiple types of handover with different handover start conditions at a time of handover in which a mobile station moves from a currently communicating base station to another base station.
2. Description of the Background Art
In the conventional mobile communication system such as PDC (Personal Digital Cellular) system that is currently providing the mobile communication service in Japan, there is only one available type of handover called hard handover.
In contrast, in the CDMA mobile communication system, the soft handover in also available in addition to the hard handover, and this soft handover further includes multiple types of soft handovers such as the same code same frequency soft handover and the different frequency soft handover. Here, these multiple types of handovers are associated with mutually different handover start conditions.
In order for each mobile station to carry out the handover properly, each base station is transmitting the perch channel (or the pilot channel). The mobile station receives the perch channel transmitted from each base station of each nearby cell, compares the receiving level of the perch channel received from each nearby cell with the receiving level of the perch channel of a currently located cell, and judges the start of the handover according to the obtained difference.
Here, as already mentioned above, the available types of handover includes the hard handover, the same code same frequency soft handover, and the different frequency soft handover, and the handover start conditions for these three types of handover are as indicated in FIG. 3. As can be seen from FIG. 3, when an influence on the radio channel capacity is taken into account, the handover start condition is weakest for the same code same frequency soft handover, second weakest for the different frequency soft handover, and strongest for the hard handover.
As already mentioned above, the conventional mobile communication system has been designed to use only one type of handover, so that there has been no means for judging an appropriate type of handover to be executed out of multiple types of handover.